control number of v8 contexts independently from number of server threads

added command-line option `--javascript.v8-contexts` to control the number of
V8 contexts created in arangod.

Previously, the number of V8 contexts was equal to the number of server threads
(as specified by option `--server.threads`). However, it may be sensible to
create different amounts of threads and V8 contexts. If the option is not
specified, the number of V8 contexts created will be equal to the number of
server threads. Thus no change in configuration is required to keep the old
behavior.